Basically chmod (aka "permissions") is short for "change mode". On a linux server such as ours it basically is how you change permissions for files and directories that can be used by a script. And also what can be done with those files and directories. There are three parts to a chmod number. The first number is for "owner", the second number is for "group", and "others" is the third. Most scripts will give you the chmod settings that will be needed in either the "readme" for the script or in the script code itself. To give you an example of the meaning of owner, groups, and others, lets use the ZboX web forum script as an example. The definition of the three in the forum are:
Owner: Me. I have access to everything in the script including admin
areas.
Group: Everyone who has registered for the forum. You can read and
execute certain aspects of the script like posting.
Others: This would be the unregistered "Guests" All they can do is read
the forum. They can't post or interact in any way.
Below are examples of the chmod numbers and their useage.
777: all can read/write/exec
755: owner can do all, group/others can read/exec
644: owner can read/write, group/others can read only
For Directories:
777: all can read, write, search
755: owner can do all, others and group can only search
Typical settings:
cgi scripts: 755
Data files: 666
Configuration files: 644 (files not updated by scripts)
Directories: 777 (with proper permissions on files in directory)
Here's a security tip: chmod a script to 400 if you want to disable it. You can always chmod it to 755 when you want to run it again.
Here's a handy little script that allows you to figure what chmod number you need:
http://javascriptkit.com/script/script2/ch...chmodcal2.shtml 
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